Novel, three-dimensional integrated circuits require reliable methods for handling of thin wafers in order to be able to successfully transport the thin wafers through the necessary product processes on the wafer back. The method of temporary bonding had become established in past years. In it the product wafer with a fully or partially finished first main surface is mounted on a carrier by means of a suitable method, especially by means of adhesive technology. Here this first main surface points in the direction of the carrier wafer. The product wafer is then thinned by means of known grinding techniques. After this thinning process other production steps are carried out on the back of the thin wafer. In the past, processes in which high thermal stresses were produced in the wafer, such as for example abrupt heating and/or cooling, led to problems. Often the wafer acquired dimples which made further processing impossible. The dimples are at the same time sites on which the adhesive used for fixing of thin wafers runs and thus the adhesive thickness is not uniform,
Wafer thinning on backgrind tapes (BG tape), therefore not stable carrier substrates, is known. Here the wafer is usually only thinned by means of grinding methods. There is no further working on the wafer back. At least in this case complex structures such as wiring lines or the like are no longer produced. In this region it is conventional to thin the wafers by means of a succession of coarse and fine grinding processes. These grinding processes however generally leave damage to the crystal structure on the ground wafer surface. This damage leads to stresses. Therefore, in this domain possibilities for eliminating this damaged layer have been researched in recent years. The result is so-called “stress relief processes”. But in order to be able to bypass these processes, manufacturers of grinding systems and grinding tools such as for example the Disco company in Japan have also worked on grinding wheels which eliminate the necessity of stress relief. One very popular product in this domain is for example the so-called Polygrind grinding wheel which makes it possible to saw the wafers immediately after thinning and to deliver them into the final chip packaging (called “packaging” in the industry).
The second relevant domain is the domain of thinning of wafers which are mounted on rigid carrier substrates. In this domain the wafers are likewise thinned to the desired target thickness by means of coarse and fine grinding methods. Typically target thicknesses of less than 100 μm are desirable. But recently wafers are being thinned preferably to 75 or 50 μm. It is expected in the future that the wafers will be more radically thinned to 30, 20 or even 10 μm. In this range the detailed process sequence in thinning of the wafer has been conventionally determined by the necessary surface quality. Often the backthinning process ends with the use of fine grinding processes employing the Polygrind grinding wheels. In this domain to date intentionally chosen processes have not been used to improve the surface quality for further working, especially in thermal applications. This is among others also the case because the rigid carrier was regarded as a sufficient means to adequately support and keep flat the thin wafer during the following process.
Therefore the object of this invention is to devise a device and a method with which further handling is facilitated or enabled with increasingly thinner, temporarily fixed product wafers, especially for following chemical processes.
This object is achieved with the features of the claims. Advantageous developments of the invention are given in the dependent claims. All combinations of at least the two features given in the specification, the claims and/or the figures also fall within the framework of the invention. At the given value ranges, values within the indicated limits will also be disclosed as boundary values and will be claimed in any combination.